The invention relates to a new method of dehydrocyclizing alkanes, more particularly dehydrocyclizing alkanes comprising paraffins containing at least 6 carbon atoms, to form the corresponding aromatic hydrocarbons.
Catalytic reforming is well known in the petroleum industry and refers to the treatment of naphtha fractions to improve the octane rating. The more important hydrocarbon reactions occurring during reforming operation employing catalysts comprising dehydrogenation-promoting metal components include dehydrogenation of 6-ring naphthenes and dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of normal paraffins to isoparaffins, dealkylation of alkylbenzenes, and hydrocracking of relatively long-chained paraffins. Hydrocracking reactions which produce high yields of light gaseous hydrocarbons, e.g., methane and ethane, are to be particularly avoided during reforming as this decreases the yield of gasoline boiling products. Furthermore, since hydrocracking is an exothermic process, as contrasted to reforming which, in general, is endothermic, hydrocracking reactions which result in the production of high yields of light gaseous products are generally accompanied by severe temperature excursions which can result in temperature runaways in a reforming operation.
Dehydrocyclization is one of the main reactions in the reforming process. The conventional methods of performing these dehydrocyclization reactions are based on the use of catalysts comprising a noble metal on a carrier. Known catalysts of this kind are based on alumina carrying 0.2% to 0.8% by weight of platinum and preferably a second auxiliary metal.
The possibility of using carriers other than alumina has also been studied and it was proposed to use certain molecular sieves such as X and Y zeolites, which appeared suitable provided that the reactant and product molecules were sufficiently small to pass through the pores of the zeolite. However, catalysts based upon these molecular sieves have not been commercially successful.
In the conventional method of carrying out the aforementioned dehydrocyclization, hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of 430.degree. C. to 550.degree. C. and pressures of 100 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The rate of conversion of the hydrocarbons into aromatic hydrocarbons varies with the reaction conditions and the nature of the catalyst.
The catalysts hitherto used have given satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction; can be used to improve the rate of conversion to aromatic hydrocarbons without requiring higher temperatures and lower pressures, which usually have a considerable adverse effect on the stability of the catalyst; and produce excellent results with C.sub.6 -C.sub.8 paraffins, but run length and regenerability are problems and satisfactory regeneration procedures are not known.
In one method of dehydrocyclizing aliphatic hydrocarbons, hydrocarbons are contacted in the presence of hydrogen at a temperature of 430.degree. C. to 550.degree. C. with a catalyst consisting essentially of a type L zeolite having exchangeable cations of which at least 90% are alkali metal ions selected from the group consisting of ions of sodium, lithium, potassium, rubidium and cesium and containing at least one metal selected from the group which consists of metals of Group VIII of the Periodic Table of Elements, tin and germanium, said metal or metals including at least one metal from Group VIII of said Periodic Table having a dehydrogenating effect, so as to convert at least part of the feedstock into aromatic hydrocarbons.
A particularly advantageous embodiment of this method is a platinum/alkali metal/type L zeolite catalyst because of its excellent activity and selectivity for converting hexanes and heptanes to aromatics, but run length and regenerability remain a problem.